The present invention relates generally to current probes and more particularly to a current probe suitable for detecting wide bandwidth large currents from DC to high frequencies using an orthogonal fluxgate magnetic sensor and a Rogowski coil.
A current probe is used to detect a current signal flowing in a line under test, such as a cable. A current probe using a transformer is suitable for detecting a relatively weak current in the micro or milliamp range. However, for high currents in the kilo or mega-amps, a current detecting circuit using a Rogowski coil would be suitable. The Rogowski coil does not require the use of a magnetic core and thus the size is smaller than a corresponding transformer rated for such currents.
FIG. 1 shows a schematic block diagram of a current detecting circuit using the Rogowski coil. The Rogowski coil 12 has a detecting coil 11 and a conducting return wire 9 formed from a single piece of wire 13. The wire is formed in loops to produce the detecting coil 11. The return wire 9 has one end 9b connected to the end loop of the detecting coil 11 at 12b and is folded back through the center of the detecting coil 11 to the beginning loop 12a of the detecting coil 11. The beginning loop 12a of the detecting coil 11 and the free end 9a of the return wire 9 are closely spaced together. In addition, the ends 12a and 12b of the detecting coil 11 are also arranged to be physically close together so that the detecting coil 11 constitute a magnetic closed loop around a line under test 10. The beginning loop 12a of the detecting coil 11 is coupled to the integration circuit that has a resister 14, capacitor 16 and an operational amplifier 18. The free end 9a of the conducting return wire 9 is coupled to electrical ground.
A current Ip flowing in the line under test 10 generates magnetic flux that induces a voltage in the Rogowski coil 12. The integration circuit maintains a flat frequency characteristic by lowering the gain of the circuit as the frequency of the current Ip increases, which increases the induced voltage in the Rogowski coil 12.
Another reason why the Rogowski coil is suitable for detecting large currents is its flexibility. Large currents are sometime carried by a thick metal bus bar. If the metal bus bar is intricately wired, a desired line under test of the bus bar may not be in a position for putting it through a detecting portion of the current probe. A flexible Rogowski coil can make a loop around the line under test by passing it between wires. A drawback to the Rogowski coil, however, is that it cannot measure DC current and has low sensitivity at low frequency.
A magnetic sensor may be used to measure a current value from DC to low frequencies. A Hall element, magneto-resistive (MR) element, giant magneto-resistive (GMR) element, fluxgate element and the like are known as the magnetic sensors. They provide a voltage according to a magnetic field so that it makes it possible to detect a current from DC to low frequencies by detecting the voltage.
The fluxgate element has parallel and orthogonal types. U.S. Pat. No. 6,380,735 (Kawakami) discloses a magnetic sensor of an orthogonal fluxgate element. FIG. 2 shows a schematic diagram of the orthogonal fluxgate element. A conductive bar 21 is placed coaxially through a cylindrical magnetic core 22. The conductive bar 21 is coupled to a high-frequency power source 25 that provides a high frequency signal. A detecting coil 23 is placed around the magnetic core 22 for detecting the magnetic flux of a magnetic field under test. FIG. 3 illustrates the magnetic flux distribution formed by an exciting current IEX flowing through the conductive bar 21. If the longitudinal directions of the conductive bar 21 and the core 22 are aligned in parallel with the magnetic field under test, the magnetic field under test is attracted toward the core 22 so that the magnetic flux is formed through the core 22 as shown in FIG. 4a. 
When a driving current IEX of a sine wave as shown in FIG. 5(1) is provided to the conductive bar 21, the circumferential surface of the core 22 is magnetized as indicated by arrows as shown in FIGS. 4b or 4d. If the driving current is increasing from the value shown in FIG. 5(1a) to the maximum value shown in FIG. 5(1b), the magnetization the core 22 reaches a saturated state so that the magnetic flux of the magnetic field under test leaves the core 22 and is aligned parallel with the conductive bar 21. During this period, the degree of the magnetization of the core 22 in the longitudinal direction is decreasing as shown in FIG. 5(2). The output voltage of the detecting coil 23 is larger at the position where the changing rate of the magnetization of the core 22 in the longitudinal direction is faster.
As the driving current IEX is decreasing to the zero-crossing point shown in FIG. 5(1c) from the maximum value shown in FIG. 5(1b), the magnetic flux of the magnetic field under test again passing through the core 22 as shown in FIG. 5(2b to 2c). When the direction of the driving current IEX is reversed, the circumferential face of the core 22 is magnetized in a reverse direction. When the driving current reached the maximum reversed value, the magnetization of the core 22 again reaches a saturated state and the magnetic flux of the magnetic filed under test again leaves the core 22 and is aligned parallel with the longitudinal direction of the core 22 as shown in FIG. 4d. As described, the direction and the density of the magnetic flux changes according to the change of the driving current, which induces the voltage of the detecting coil 23, wherein the voltage of the detecting coil 23 has 2 cycles of the driving current.
As described, the flexibility of the Rogowski coil is suitable for measuring a large current, but has low sensitivity for a current of DC or low frequency. Therefore, what is desired is to provide a flexible current probe suitable for seamlessly detecting large currents from DC to high frequency.